The invention concerns a procedure for transmission of variable-format images over low data rate transmission systems. The invention is applicable, for example, to videophone systems.
The international standard CCITT H.261 gives the specifications of a codec (coder-decoder) of video signals for audiovisual services with a data rate of p.times.64 Kbits/s. This standard prescribes in particular a source image format known as QCIF, which corresponds to a resolution of 144 lines of 176 pixels. To reduce the throughput of transmitted data, a compression process based on the following steps is used: prediction based on movement estimation, mapping into the frequency domain using a discrete cosine transformation (DCT), quantization of the frequency coefficients and coding of these coefficients using a variable-length code.
The DCT transform is applied to luminance blocks of 8 pixels by 8 pixels. For the estimation of movement and for transmission, blocks are grouped into macroblocks containing 4 adjacent luminance blocks totaling 16 pixels by 16 pixels. In this way an image in QCIF format includes 9 macroblocks vertically and 11 macroblocks horizontally.
Videophones currently on the market also use other image formats, specifically formats of 112 lines of 128 pixels and 96 lines of 128 pixels, respectively known as QCTX and MVTS.